There is no denying the ease and efficiency an electric vehicle represents. Electric motors are efficient and quiet. Batteries can be recharged when diurnal demands on the grid are lowest. An electric vehicle is essentially off when idle, and then seamlessly back on again when needed. Required maintenance is minimal. And electric vehicles emit no exhaust of any form.
But there are drawbacks to pure electric vehicles. Batteries are heavy and have a relatively low energy density compared to liquid fuels. Electric vehicles have a relatively limited range (from 30–200 miles). Recharging the batteries, the equivalent of refueling, is very slow and may require 8–12 hours for a discharged battery to be fully recharged. While electric powered vehicles have excellent torque capabilities and good acceleration, they generally lack the ability to cruise efficiently at high speed.
Hybrid vehicles that use both a gasoline and an electric motor are an ideal way to take advantage of the best qualities of gasoline and electric powered vehicles. Generally, hybrids are configured so that a gasoline engine powers the wheels directly, and/or powers a generator to produce electricity that powers the electric motor directly or is stored in the batteries. The gasoline engine, the electric motor, and the batteries can all be downsized resulting in a significant weight reduction. Depending on the desired vehicle performance, efficiency, etc, the gasoline and electric engines may be sized and configured to run singly or together depending on a number of factors. Such hybrid vehicles and their control strategies are disclosed in U.S. Pat. Nos. 6,540,035 B2, and 6,491,120 B1, the contents of which are specifically incorporated by reference herein.
A newer type of hybrid vehicle combines a high efficiency fuel cell with an electric motor and a battery pack. Fuel cell hybrids essentially provide two sources of electricity for a main drive electric motor: the fuel cell stack and electricity stored in the batteries. Fuel cell hybrids have a number of advantages over gasoline hybrids such as very few moving parts and significantly reduced or no emissions. Fuel cell hybrids are discussed in U.S. Pat. No. 6,580,977 B2, for example, the contents of which are specifically incorporated by reference herein. Some consider them superior to gasoline hybrids. Fuel cell hybrids like internal combustion hybrids are designed so that the battery pack is recharged as the vehicle operates (using current generated from operation of the fuel cell and/or current generated via braking, etc.), thus eliminating the need to “plug in” the battery to recharge it.
Like electric vehicles, battery technology is a limiting factor for hybrid vehicles as well. The solution to this problem involves cost and performance tradeoffs. At the high end of the cost scale are maintenance free (i.e. sealed, starved electrolyte), high energy density, and long life batteries. At the other end of the cost scale are non-maintenance free (i.e. watered, liquid electrolyte), lower energy density, and short lived batteries. The most useful batteries are obviously ones that are somewhere between these the two extremes, that is batteries that have reasonable energy density that are not too expensive.
The number of possible mid-range choices, however, has been limited by the necessity of using maintenance free batteries. This is because requiring the regular addition of water or pre-mixed electrolyte to flooded cells is fraught with difficulties. Some examples of potential problems are the following: 1) Both acid and base electrolyte (depending on battery chemistry) are dangerous chemical solutions and the necessity of regularly adding water or electrolyte to such solutions carries an increased risk of significant harm. 2) Improper addition of water electrolyte could destroy a battery pack, a $1,500 to $7,000 replacement cost. And, 3) required regular watering destroys the “maintenance free” aspect of a hybrid vehicle.